1. Field of the Invention
The present invention relates, generally, to an active pixel sensor, and, more particularly, to an active pixel sensor, that combines nonlinear capacitance characteristics with logarithmic characteristics, thereby having a high dynamic range, compared to the existing pixel sensors.
2. Description of the Related Art
Currently, image sensors are used in various fields, such as mobile device, camera, surveillance camera, and car camera fields, and the image sensors are required to have a relatively wide dynamic range. Therefore, image sensors employ a logarithmic pixel structure, in which voltage follows a log function in response to incident light. However, the logarithmic pixel structure has characteristics in which response to light is slow when light is weak, and the total output voltage range is narrow, that is, the total output voltage range corresponds to a few millivolts (mV), and thus the logarithmic pixel structure is vulnerable to noise.
In order to overcome theses problems, a linear-logarithmic structure has been proposed. In this structure, a linear response appears when light is weak, and a logarithmic response appears when light is strong. However, this structure also has defects in that an image may appear unnatural because the response suddenly switches from a linear response to a logarithmic response, and in that the photodiode characteristics for a linear section are not optimal.
In particular, in the case of the existing linear-logarithmic pixels, a dynamic range of about 140 dB is realized when a voltage of 3.3 V is used. That is, the dynamic range for the linear section is about 60 dB, and the dynamic range for the logarithmic response is about 80 dB. In such a pixel, when the supply voltage drops, the output voltage range for the linear section is narrow, and thus the dynamic range decreases. Further, since the output voltage range for the linear section has a major effect on the total Signal-to-Noise Ratio (SNR), there is a defect in that the SNR is low. In addition, since a photodiode uses a PN junction, which forms a linear section, there is a defect in that it is vulnerable to noise and the linear dynamic range is narrow (refer to “Combined Linear-Logarithmic Image Sensor” by G. G. Strom in ISSCC 2004).
Further, a pixel structure using nonlinear capacitance is used as another method of increasing dynamic range. In this structure, a photodiode is surrounded with a photogate, so that dark current flowing through the photodiode is decreased and responsivity is increased. In addition, when the dynamic range is increased using the capacitance of a photogate, the dynamic range is 75 dB.
Currently, since the dynamic range of a well-manufactured Charge Coupled Device (CCD) is 72 dB, the above-described pixel structure does not have a significant industrial advantage in view of the dynamic range. Accordingly, only when the dynamic range is equal to or greater than 120 dB, which is almost the same as the dynamic range of human eyes, is the advantage of a high dynamic range pixel obtained. Further, in the case of the existing pixel, an additional bias voltage should be applied to a photogate. Therefore, the fill factor is decreased due to an additional voltage line.